Conditions for Barrel and Clam-Shell Liquid Drops to Move on Bio-inspired Conical Wires

Abstract

It has been reported that, in a foggy environment, water drops with either barrel or clam-shell shapes are capable of self-running on conical wire-like structures, such as cactus spines, spider silk, and water striders’ legs. On the other hand, the corresponding moving mechanisms are still not quite understood. For instance, it is unclear under what conditions clam-shell drops would move from the tip towards the root on a conical wire. In this work, based on the balance of forces, we derive conditions for a drop to self-transport towards or away from the root. We find that, although barrel and clam-shell drops have different shapes, these conditions are applicable to both of them, which thus provide good guidelines for developing artificial fog collectors. Furthermore, based on the derived conditions, we interpret drop movements on both hydrophilic and hydrophobic wires, with the support of experimental results on cactus spines. Finally, our results indicate that not all the cacti are able to harvest water from fog.

Bioinspired Azimuthally Varying Nanoscale Cu Columns on Acupuncture Needles for Fog Collection

“…It should be mentioned that the droplet movement on a conical wire due to a geometrical gradient has already been reported in the literature, ,,, while rapid water movement on a cylindrical-shaped needle (BIDn) has not been reported yet. − …”

“…In fact, the fabricated asymmetrical gradient in the needle exhibits excellent fog collection under blowing fog flow on the needle. The cactus spines exhibit similar behavior in nature where they will capture fog droplets due to the existence of a Laplace pressure difference . The tendency of droplets to move toward more wettable surfaces leads to instability of the droplets located at the superhydrophobic regions.…”

“…The cactus spines exhibit similar behavior in nature where they will capture fog droplets due to the existence of a Laplace pressure difference. 46 The tendency of droplets to move toward more wettable surfaces leads to instability of the droplets located at the superhydrophobic regions. This instability facilitates the water's transport by diffusion and releasing tiny droplets' surface energy.…”

Self-propulsion of Leidenfrost Drops between Non-Parallel Structures

“…At room temperature, a capillary force on a liquid drop may be created using an asymmetric structure, such as a conic capillary tube 5 , a conic fiber 6 – 10 , a pair of non-parallel plates 11 – 15 , or a spiral surface 16 . These structures have varied radii 5 – 10 , gaps 11 – 15 , or surface curvatures 16 along their longitudinal directions. Accordingly, a gradient of Laplace pressure is produced inside a drop, resulting in a directional motion of the drop on such a structure.…”

“…That is, can they also be applied to control Leidenfrost drops? Due to non-wetting property of a Leidenfrost drop, conic fibers 6 – 10 may not be a good option, since the Leidenfrost drop may not stick to their surfaces. The same applies to a spiral surface 16 .…”